A center electrode of a spark plug, in general, includes a flange-like large diameter portion at a rear end side portion of the center electrode. The leading end side of the large diameter portion includes a barrel portion that is smaller in diameter than the large diameter portion and a small diameter portion that is smaller in diameter than the barrel portion. Heretofore, when manufacturing this kind of center electrode having multi-step diameters, a cylindrical electrode member is first prepared, and then the barrel portion is formed by an extrusion process, after which a small diameter portion is formed at the leading end portion of the barrel portion by an extrusion (for example, refer to JP-A-8-213150).
However, depending on a difference between the diameter of the barrel portion and the diameter of the small diameter portion, the barrel portion may bulge in a radial direction due to a pressure applied to the rear end of the electrode member by a punch when extruding the small diameter portion.